LTC1250 Linear Technology, LTC1250 Datasheet - Page 7

• Manufacturer Part Number: LTC1250
• Description: Very Low Noise Zero-Drift Bridge Amplifier
• Manufacturer: Linear Technology

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Output Drive
The LTC1250 includes an enhanced output stage which
provides nearly symmetrical output source/sink currents.
This output is capable of swinging a minimum of 4V into a
1k load with 5V supplies, and can sink or source >20mA
into low impedance loads. Lightly loaded (R
LTC1250 will swing to within millivolts of either rail. In single
supply applications, it will typically swing 4.3V into a 1k load
with a 5V supply.
Minimizing External Errors
The input noise, offset voltage, and bias current specs for the
LTC1250 are all well below the levels of circuit board
parasitics. Thermocouples between the copper pins of the
LTC1250 and the tin/lead solder used to connect them can
overwhelm the offset voltage of the LTC1250, especially if a
soldering iron has been around recently. Note also that when
the LTC1250’s output is heavily loaded, the chip may
dissipate substantial power, raising the temperature of the
package and aggravating thermocouples at the inputs.
Although the LTC1250 will maintain its specified accuracy
under these conditions, care must be taken in the layout to
prevent or compensate circuit errors. Be especially careful
of air currents when measuring low frequency noise; nearby
moving objects (like people) can create very large noise
peaks with an unshielded circuit board. For more detailed
explanations and advice on how to avoid these errors, see
the LTC1051/LTC1053 data sheet.
Sampling Behavior
The LTC1250’s zero-drift nulling loop samples the input at
aliasing. Signals above this frequency may show aliasing
behavior, although wideband internal circuitry generally
keeps errors to a minimum. The output of the LTC1250 will
have small spikes at the clock frequency and its harmonics;
these will vary in amplitude with different feedback configu-
rations. Low frequency or band-limited systems should not
be affected, but systems with higher bandwidth
(oversampling A/Ds, for example) may need to filter out
these clock artifacts. Output spikes can be minimized with a
large feedback capacitor, but this will adversely affect noise
performance (see Input Capacitance and Compensation on
5kHz, allowing it to process signals below 2kHz with no
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Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
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tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
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Single Supply Operation
The LTC1250 will operate with single supply voltages as low
as 4.5V, and the output swings to within millivolts of either
supply when lightly loaded. The input stage will common
mode to within 250mV of ground with a single 5V supply,
and will common mode to ground with single supplies
above 11V. Most bridge transducers bias their inputs above
ground when powered from single supplies, allowing them
to interface directly to the LTC1250 in single supply applica-
tions. Single-ended, ground-referenced signals will need to
be level shifted slightly to interface to the LTC1250’s inputs.
Fault Conditions
The LTC1250 is designed to withstand most external fault
conditions without latch-up or damage. However, unusually
severe fault conditions can destroy the part. All pins are
protected against faults of 25mA or 5V beyond either
supply, whichever comes first. If the external circuitry can
exceed these limits, series resistors or voltage clamp diodes
should be included to prevent damage.
The LTC1250 includes internal protection against ESD dam-
age. All data sheet parameters are maintained to 1kV ESD on
any pin; beyond 1kV, the input bias and offset currents will
increase, but the remaining specs are unaffected and the
part remains functional to 5kV at the input pins and 8kV at the
output pin. Extreme ESD conditions should be guarded
against by using standard anti-static precautions.
the previous page). Applications which require spike-free
output in addition to minimum noise will need a low-pass
filter after the LTC1250; a simple RC will usually do the job
(Figure 4). The LTC1051/LTC1053 data sheet includes more
information about zero-drift amplifier sampling behavior.
Figure 4. RC Output Pole Limits Bandwidth to 330Hz
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